Image processing apparatus, image processing method, and computer-readable recording medium

ABSTRACT

Disclosed herein is an image processing apparatus including: a screen processor for determining each pixel value of a screen processing application unit region composed of a plurality of pixels to which a threshold matrix is applied, in an inputted image; and for carrying out a screen processing by applying one threshold matrix among a plurality of threshold matrices in which applied positions of the threshold values are different from each other, to the plurality of pixels in the screen processing application unit region based on each determined pixel value.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an image processing apparatus, an imageprocessing method, and a computer-readable recording medium for carryingout a screen processing for an inputted image by applying a thresholdmatrix to the inputted image.

2. Description of Related Art

A screen processing is a processing in which a threshold matrix in whichdifferent threshold values are set in a matrix manner (hereinaftersimply referred to as matrix), is used to carry out binary (ormultilevel) operation for a multilevel image data and to reproduce ahalftone. Conventionally, various methods for improving a gradationcharacteristic have been suggested. One of such methods is a method forreproducing a halftone depending on the density of an inputted image.For example, there is a conventional method in which a matrix size canbe changed depending on the change of the density of an inputted imageto set threshold values in the matrix having the changed size (seeJapanese Patent Unexamined Publication No. 5-199406 for example).According to this method, in parts of an image in which the densitychange is small, the matrix size is expanded and the same thresholdvalues are applied in order to improve the processing efficiency, and inparts of an image in which the density change is large, the matrix sizeis reduced and different threshold values are applied in order toimprove the gradation characteristic.

There is another method in which a matrix to be applied is changeddepending on the density of an inputted image (for example, see JapanesePatent Unexamined Publication No. 9-107473). According to this method,in case of high density, a matrix in which threshold values are arrangedin a whorl-like manner so that threshold values are increased from thecenter of the matrix is used, and in case of low density, a matrix inwhich the above arranged threshold values are inverted is used. In themethod, it is possible to reproduce a halftone depending on the changein the density of an inputted image by a simple matrix.

However, in a conventional screen processing, when the matrix in whichthe applied positions of the threshold values are fixed is used, somearrays of the pixel values of an inputted image may cause deterioratedreproducibility of a thin line having a width of one to several pixels.

For example, as shown in FIG. 7, the case where a 3 pixels×1 pixelmatrix having an array of threshold values of (0, 85, 170) is applied toan inputted image having a pixel value array (160, 0, 0) in order toobtain a multilevel image, is considered. In this case, when “the pixelvalue>the threshold value” is established, the pixel value is directlyoutputted as an output value. When “the pixel value≦the threshold value”is established, an output value of “0” is outputted. In the case of theinputted image shown in FIG. 7, a threshold value of “0” which issmaller than a pixel value of “160” is applied to the pixel value of“160” and thus the output value is “160”. Thus, no problem is causedbecause the inputted image is directly reproduced even when the matrixis applied to the pixel value.

On the other hand, another case where the same matrix is applied to aninputted image having a pixel value array of (0, 0, 160), is considered.In this case, a threshold value of “170” is applied to a pixel value of“160” as shown in FIG. 8. As a result, an output value is “0” and dataof “160” of the inputted image is lost. This causes an outputted thinline including a thin spot or the like. As a result, the reproducibilityof the inputted image is deteriorated.

SUMMARY

An object of the present invention is to improve the reproducibility ofan inputted image.

To achieve at least one of the above-described objects, an imageprocessing apparatus reflecting one aspect of the invention, comprises:

a screen processor for determining each pixel value of a screenprocessing application unit region composed of a plurality of pixels towhich a threshold matrix is applied, in an inputted image; and forcarrying out a screen processing by applying one threshold matrix amonga plurality of threshold matrices in which applied positions of thethreshold values are different from each other, to the plurality ofpixels in the screen processing application unit region based on eachdetermined pixel value.

An image processing method reflecting one aspect of the invention,comprises:

determining each pixel value of a screen processing application unitregion composed of a plurality of pixels to which a threshold matrix isapplied, in an inputted image; and

carrying out a screen processing by applying one threshold matrix amonga plurality of threshold matrices in which applied positions of thethreshold values are different from each other, to the plurality ofpixels in the screen processing application unit region based on eachdetermined pixel value.

A computer-readable recording medium reflecting one aspect of theinvention, wherein a program causes a computer to function as:

a screen processor for determining each pixel value of a screenprocessing application unit region composed of a plurality of pixels towhich a threshold matrix is applied, in an inputted image; and forcarrying out a screen processing by applying one threshold matrix amonga plurality of threshold matrices in which applied positions of thethreshold values are different from each other, to the plurality ofpixels in the screen processing application unit region based on eachdetermined pixel value.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will become more fully understood from thedetailed description given hereinafter and the accompanying drawinggiven by way of illustration only, and thus are not intended as adefinition of the limits of the present invention, and wherein:

FIG. 1 is a view showing an internal structure of an image processingapparatus in this embodiment;

FIG. 2 is a view showing an image processor and a printer unit of FIG.1;

FIG. 3 is a view showing an example of a threshold matrix;

FIG. 4 is a flowchart showing the flow of the processing in a screenprocessor;

FIG. 5 is a view showing an inputted image and an output image after ascreen processing using a matrix;

FIG. 6 is a view showing a screen processor in another embodiment;

FIG. 7 is a view for explaining a conventional screen processing and theoutput result thereof; and

FIG. 8 is a view for explaining a conventional screen processing and theoutput result thereof.

DETAILED DESCRIPTION OF THE PREFEREED EMBODIMENT

Hereinafter, embodiments according to an image processing apparatus, animage processing method, and a recording medium of the present inventionwill be described with reference to the drawings.

First, the structure will be described.

FIG. 1 illustrates an internal structure of an image processingapparatus 100 in this embodiment.

As shown in FIG. 1, the image processing apparatus 100 comprises a mainbody 10, an image reading unit 20, an operation unit 30, a touch panel40, a display unit 50, and a printer unit 60. The main body 10 comprisesan image processor 1, a control unit 2, a storage unit 3, a dynamicrandom access memory (DRAM) control unit 4, a DRAM 5 and the like.

The image reading unit 20 comprises a light source, a charge coupleddevice (CCD), an A/D converter and the like. When the image reading unit20 reads an image, a document is irradiated with light from the lightsource to scan the document and the light is reflected. An image isformed by the reflected light. A document image is read by carrying outa photoelectric conversion for the image with the CCD to generate animage signal (analog signal). Then, the image signal is converted todigital image data by an A/D converter and the converted data isoutputted to the image processor 1 of the main body 10. The term “image”herein includes not only an image of a figure or photograph or the likebut also a character image, such as image of letters and marks, or thelike.

In the image reading unit 20, an image is separated from each color ofred (R), green (G), and blue (B) to output each color image data. Eachtype of processing, such as a processing for correcting brightnessvariation, a processing for correcting a brightness characteristic, avariable power processing, a γ conversion processing (a processing forconverting a brightness-linear characteristic to a density-linearcharacteristic), a color correction processing or the like, is carriedout for each color image data by a correction processor (not shown)provided between the image reading unit 20 and the image processor 1.Then, the color image data is inputted to the image processor 1.

The operation unit 30 comprises various function keys (e.g., a start keyfor instructing the start of a printing operation, a numeric key and thelike). When these function keys and the touch panel 40 are operated, theoperation unit 30 outputs a corresponding operation signal to thecontrol unit 2.

The display unit 50 comprises a liquid crystal display (LCD) integratedwith the touch panel 40 to display various operation screens foroperating a printing operation, on the LCD.

The printer unit 60 prints the image by the electrophotographic methodbased on the image data inputted from the image processor 1 of the mainbody 10. In the electrophotographic method, a photoconductive drum isirradiated with laser light from a laser light source to carry out theexpose. Thereby, an electrostatic latent image is formed. A toner imagewhich is obtained by injecting the toner from a development unit to theelectrostatic latent image, is transferred onto a recording paper toform an image. In this embodiment, four color materials of yellow (Y),magenta (M), cyan (C), and black (K) are used to print an image. Wheneach color image data is inputted to the printer unit 60, the printerunit 60 carries out a frequency modulation and a pulse width modulation(PWM) conversion by using a frequency modulation/PWM conversionprocessor 61 for the image data and inputs a modification signal to anLD driver 62. Based on the inputted modification signal, the LD driver62 actuates the laser light source to control laser light emitted fromthe laser light source, that is, light exposure.

Next, the each section of the main body 10 will be described.

In accordance with various control programs stored in the storage unit3, such as a system program, a print processing program and the like,the control unit 2 controls the operations of each section of the imageprocessing apparatus 100 in a concentrated manner.

The storage unit 3 stores various control programs executed by thecontrol unit 2 and parameters and data and the like, required for theeach section. For example, the storage unit 3 stores data for a matrixto be used in a screen processing by the image processor 1.

The DRAM control unit 4 controls an input and an output of image datastored in the DRAM 5. The DRAM 5 is an image memory for storing imagedata.

Next, the image processor 1 according to the present invention will bedescribed.

As shown in FIG. 2, the image processor 1 comprises an imagedetermination unit 11, a color converter 12, an averaging processor 13,a γ correction processor 14, and a screen processor 15 and the like. Theaveraging processor 13, the γ correction processor 14, and the screenprocessor 15 are provided for each color of Y, M, C, and K.

The image determination unit 11 analyzes the inputted image data read bythe image reading unit 20 in order to determine whether each pixel hasan attribute of a character image, a line image, or a photographicimage. Then, the image determination unit 11 generates an attribute dataTAG showing the determination result for each pixel and outputs theattribute data TAG to the screen processor 15. When the attribute is acharacter attribute, the TAG is set to 0. When the attribute is a lineimage attribute, the TAG is set to 1. When the attribute is aphotographic image attribute, the TAG is set to 2.

The color converter 12 carries out a color correction for each colorimage data of R, G, and B inputted from the image reading unit 20. Then,the color converter 12 converts the image data to each color image datahaving each color material of Y, M, C, and K, which can be outputtedfrom the image processing apparatus 100. Each image data of Y, M, C, andK, which is generated by the color conversion is outputted to theaveraging processor 13.

The averaging processor 13 carries out an averaging processing for theinputted image data. Each image data having each color material, forwhich the averaging processing is carried out, is outputted to thecorresponding γ correction processor.

The γ correction processor 14 carries out a gradation processing for theinputted image data by using a lookup table (LUT) previously preparedfor γ correction in order to carry out a γ correction processing. Eachimage data having each color material, for which the γ correctionprocessing, is outputted to the corresponding screen processors 15.

The screen processor 15 carries out a screen processing for the inputtedimage data (hereinafter referred to as inputted image) by the dithermethod. In the screen processing, a matrix in which different thresholdvalues are previously set, is read out from the storage unit 3 and isapplied to the inputted image to carry out a binary or multileveloperation. In this embodiment, a case in which a screen processing forcarrying out the multilevel operation for the image, is realized by asoftware processing, will be explained.

FIG. 3 illustrates an example of a matrix.

A matrix is composed of a plurality of elements which are called cells(one cell corresponds to one pixel). In the cells, different thresholdvalues TH are set, respectively. The matrix shown in FIG. 3 is composedof 1×3 cells e1 to e3. The cells e1 to e3 are set to three thresholdvalues (0, 85, 170). Each application number is set to each thresholdvalue in ascending order of the threshold values. Specifically, anapplication number “1” is set to the smallest threshold value of “0”, anapplication number “1” is set to a threshold value “85”, and anapplication number “3” is set to a threshold value “170”.

The screen processor 15 scans the matrix as described above in a mainscanning direction and a sub scanning direction of the inputted image tocompare a pixel group corresponding to the matrix with threshold valuesin the matrix. Then, the multilevel operation is carried out.

Hereinafter, the flow of the processing in the screen processor 15 willbe explained with reference to FIG. 4.

As shown in FIG. 4, in an inputted image, with respect to a pixel groupin which the matrix is applied to each pixel (hereinafter referred to asa target pixel), the screen processor 15 firstly refers to the attributedata TAG and refers to the pixel value of each target pixel (Step S1).In case of the matrix shown in FIG. 3, which is composed of 1×3 cells,the screen processor 15 refers to pixel values of 3 pixels correspondingto the matrix applied to the inputted image.

Next, the screen processor 15 determines whether the TAG of the targetpixel group is “2” or not (Step S2). In case of TAG=2 (Step S2; Y),i.e., when the target pixel group constitutes a photographic image, thescreen processor 15 reads the matrix corresponding to the photographicimage from the storage unit 3. The matrix corresponding to thephotographic image may be a matrix that is exclusively prepared for thephotographic image and that is stored in the storage unit 3 or may be apredetermined matrix among a plurality of matrices stored in the storageunit 3. Then, the matrix is applied to the target pixel group to performa screen processing (Step S3). Specifically, pixel values of the targetpixels at corresponding positions in the applied matrix are comparedwith threshold values. In case of “pixel values>threshold values”, thepixel values are directly outputted as an output value. In case of“pixel values≦threshold values”, an output value “0” is outputted and amultilevel operation is performed.

On the other hand, when TAG is not “2” (Step S2; N) (i.e., when thetarget pixel group constitutes a character image or a line image), thescreen processor 15 numbers the pixels in the target pixel group indescending order of the pixel values (Step S4). For example, when pixelsof the referred inputted image are arranged in an order of pixel valuesof “0”, “0”, and “160” as shown in FIG. 5, the maximum pixel value is“160” and thus the target pixel “160” is numbered as “1”. The remainingtwo pixels having the same pixel value “0” are arbitrarily numbered as“2” and “3”.

Next, the screen processor 15 applies the matrix to the target pixelgroup.

First, the screen processor 15 decides a matrix to be applied to thetarget pixel group. The screen processor 15 selects a matrix in which anarray of numbers assigned to the target pixel group corresponds to anarray of numbers assigned to threshold values of the matrix (Step S5).In case of an example of FIG. 5, the screen processor 15 selects thematrix that has an array of numbers of “321”, that is, the array inwhich the threshold value “170” numbered as “3”, the threshold value“85” numbered as “2”, and the threshold value “0” numbered as “1” arearranged). The matrix also may be selected in such a manner that thestorage unit 3 stores all matrices previously prepared by combining theapplied positions of the threshold values and then a suitable matrix isread out based on an array of numbers assigned to threshold values ofthe matrix. Alternatively, whenever the processing is performed, amatrix also may be prepared by applying threshold values to cells e1 toe3 of a matrix so that an array of numbers assigned to pixel values of atarget pixel group corresponds to an array of numbers assigned tothreshold values.

The screen processor 15 applies the selected matrix to a target pixelgroup to carry out the target pixel group (Step S6). Then, theprocessing is finished. The target pixels are numbered in a descendingorder of pixel values and the threshold values of the matrix arenumbered in an ascending order of threshold values. The matrix isselected so that the arrays of the numbers correspond to each other.Thus, as shown in FIG. 5, the minimum threshold value (a threshold valueof “0” in FIG. 5) is always applied to a target pixel having the maximumpixel value (a target pixel having a pixel value of “160” in FIG. 5).Specifically, there is high possibility that the target pixel having themaximum pixel value of “160” result in an output value of “160”.

The screen processor 15 shifts an applied position of the matrix in theinputted image, and repeatedly carries out the above processing at theshifted position, until the screen processing is carried out for allpixels of the inputted image.

As described above, according to this embodiment, a matrix in which theminimum threshold value is applied, is selected, and the selected matrixis applied to a target pixel having the maximum pixel value among targetpixels. Thus, the maximum pixel value of the target pixel can beretained even when the multilevel operation is carried out for thetarget pixel. The reproducibility of an original inputted image can beimproved. According to this, because in an image including thin linesand the like, a part of the thin lines can be outputted more easily, theimage of the thin line can be prevented from being deteriorated, such asthin spot. This embodiment is particularly effective.

Furthermore, the above matrix is selected only in case of TAG=0 or 1,that is, when the image is a character image or a line image. Thus, inparticular, a photographic image in which the retention of a thin lineand the like is not required, can be prevented from carrying out ascreen processing by a selected matrix.

It is noted that the above-described embodiment is a preferable exampleto which the present invention is applied and the present invention isnot limited to this.

For example, although in the description, a case in which a multileveloperation is carried out, is explained, the same processing can be alsoused in a case in which a binary operation is carried out. In the abovedescription, a relation in which an output value “0” is outputted incase of “threshold value>pixel value” and a pixel value is outputted incase of “threshold value≦pixel value”, is explained. However, a screenprocessing also can be carried out even in an opposite relation in whicha pixel value is outputted in case of “threshold value≦pixel value” andan output value “0” is outputted in case of “threshold value>pixelvalue”.

In the above description, it is explained that with respect to a targetpixel having the maximum pixel value, a matrix in which the minimumthreshold value is applied, is selected. However, with respect to atarget pixel having the maximum pixel value, because an output value isan arbitrary value except “0”, it is not necessary that the minimumthreshold value is always applied. Thus, a pixel value of a target pixelhaving the maximum pixel value also may be compared with each thresholdvalue of the matrix to select a matrix to which a threshold valuesmaller than the pixel value of the target pixel is applied. Forexample, with respect to a target pixel group of (0, 0, 160), not only amatrix to which an array of threshold values (170, 85, 0) is applied,but also a matrix to which an array of threshold values (170, 0, 85) orthe like is applied, also can be selected.

In addition, in the above embodiment, a plurality of matrices in whichthreshold values are previously set to different positions, areprovided. However, in another system, only cell elements and thresholdvalues are previously set and during a screen processing, positions ofthreshold values corresponding to each of the cell elements may be setto each unit region in which a matrix is applied, based on thecomparison result in which each pixel value of a plurality of pixels inthe unit region are compared with each threshold value thereof. In theexample of the matrix of FIG. 3, three threshold values of “0”, “85”,and “170” are prepared for the three cell elements e1 to e3. When inputpixels of “0”, “0”, and “160” shown in FIG. 5 are inputted, for example,the maximum pixel value in the pixel values is compared with eachthreshold value. Then, a threshold value which is smaller than themaximum pixel value, is set to a cell element at a positioncorresponding to the maximum pixel value.

Although in the above description, an example in which the aboveprocessing is realized by a software processing, is explained, the aboveprocessing also may be realized by hardware. In this case, the screenprocessor 15 has a structure as shown in FIG. 6. In the structure asshown in FIG. 6, a plurality of dither processing circuits a1 to a6 areprovided so as to correspond to a plurality of matrices in which theeach threshold value of “0”, “85”, and “170” is set to different appliedpositions. It is noted that arrays in parentheses shown in FIG. 6 (0,85, 170) and the like show arrays (applied positions) of thresholdvalues in matrices owned by the respective dither processing circuits a1to a6.

A selection determination unit b is provided. In the selectiondetermination unit b, the attribute data TAG and the pixel values of atarget pixel group are referred to determine an output result of any ofthe dither processing circuits a1 to a6 to be selected, and to generatea select signal. For example, with respect to the target pixel group (0,0, 160) shown in FIG. 5, a dither processing circuit a6 having a matrixin which a target pixel having the maximum pixel value corresponds tothe minimum threshold value, for example, a matrix having an array (170,85, 0) is selected and a select signal showing that the ditherprocessing circuit a6 is selected, is generated.

A selector c selects an output result of the dither processing circuitsa1 to a6 shown by the select signal inputted from the selectiondetermination unit b, among output results inputted from the respectivedither processing circuits a1 to a6, to output the result as an outputimage.

As described above, in case of a hardware configuration, all matricesare firstly applied and a screen processing is carried out tosubsequently select an output result. In this point, the hardwareprocessing is different from a software processing in which a matrix isfirstly selected to subsequently carry out a dither processing.According to the hardware configuration, because it is possible todeterminate of a pixel value and a TAG and carry out a screen processingsimultaneously, the processing time can be shortened.

The present U.S. patent application claims the priority of JapanesePatent Application No. 2006-191757 filed on Jul. 12, 2006, according tothe Paris Convention, and the above Japanese Patent Application is thebasis for correcting mistranslation of the present U.S. patentapplication.

1. An image processing apparatus, comprising: a screen processor fordetermining each pixel value of a screen processing application unitregion composed of a plurality of pixels to which a threshold matrix isapplied, in an inputted image; and for carrying out a screen processingby applying one threshold matrix among a plurality of threshold matricesin which applied positions of the threshold values are different fromeach other, to the plurality of pixels in the screen processingapplication unit region based on each determined pixel value.
 2. Theimage processing apparatus of claim 1, wherein the screen processorcompares each determined pixel value with each threshold value of thethreshold matrix, and selects one threshold matrix from among theplurality of threshold matrices based on a comparison result to applythe selected threshold matrix to the screen processing.
 3. The imageprocessing apparatus of claim 2, wherein the screen processor selects athreshold matrix in which each threshold value is applied in anascending order of the threshold values to the pixels in a descendingorder of the determined pixel values, based on the comparison result. 4.The image processing apparatus of claim 2, wherein the screen processorselects a threshold matrix in which a threshold value smaller than amaximum determined pixel value is applied to a pixel having the maximumdetermined pixel value, based on the comparison result.
 5. The imageprocessing apparatus of claim 1, further comprising: an imagedetermination unit for determining an image attribute of each pixel ofthe inputted image, wherein the screen processor carries out the screenprocessing by applying one threshold matrix among the plurality ofthreshold matrices depending on the determined image attribute.
 6. Animage processing method, comprising: determining each pixel value of ascreen processing application unit region composed of a plurality ofpixels to which a threshold matrix is applied, in an inputted image; andcarrying out a screen processing by applying one threshold matrix amonga plurality of threshold matrices in which applied positions of thethreshold values are different from each other, to the plurality ofpixels in the screen processing application unit region based on eachdetermined pixel value.
 7. The image processing method of claim 6,wherein in the carrying out of the screen processing, each determinedpixel value is compared with each threshold value of the thresholdmatrix, and one threshold matrix is selected from among the plurality ofthreshold matrices based on a comparison result to apply the selectedthreshold matrix to the screen processing.
 8. The image processingmethod of claim 7, wherein in the carrying out of the screen processing,a threshold matrix in which each threshold value is applied in anascending order of the threshold values to the pixels in a descendingorder of the determined pixel values, is selected based on thecomparison result.
 9. The image processing method of claim 7, wherein inthe carrying out of the screen processing, a threshold matrix in which athreshold value smaller than a maximum determined pixel value is appliedto a pixel having the maximum determined pixel value, is selected basedon the comparison result.
 10. The image processing method of claim 6,further comprising: determining an image attribute of each pixel of theinputted image, wherein in the carrying out of the screen processing,the screen processing is carried out by applying one threshold matrixamong the plurality of threshold matrices depending on the determinedimage attribute.
 11. A computer-readable recording medium for storing aprogram, wherein the program causes a computer to function as: a screenprocessor for determining each pixel value of a screen processingapplication unit region composed of a plurality of pixels to which athreshold matrix is applied, in an inputted image; and for carrying outa screen processing by applying one threshold matrix among a pluralityof threshold matrices in which applied positions of the threshold valuesare different from each other, to the plurality of pixels in the screenprocessing application unit region based on each determined pixel value.12. The recording medium of claim 11, wherein the screen processorcompares each determined pixel value with each threshold value of thethreshold matrix, and selects one threshold matrix from among theplurality of threshold matrices based on a comparison result to applythe selected threshold matrix to the screen processing.
 13. Therecording medium of claim 12, wherein the screen processor selects athreshold matrix in which each threshold value is applied in anascending order of the threshold values to the pixels in a descendingorder of the determined pixel values, based on the comparison result.14. The recording medium of claim 12, wherein the screen processorselects a threshold matrix in which a threshold value smaller than amaximum determined pixel value is applied to a pixel having the maximumdetermined pixel value, based on the comparison result.
 15. Therecording medium of claim 11, wherein the program further causes thecomputer to function as: an image determination unit for determining animage attribute of each pixel of the inputted image, wherein the screenprocessor carries out the screen processing by applying one thresholdmatrix among the plurality of threshold matrices depending on thedetermined image attribute.
 16. An image processing apparatus,comprising: a screen processor unit which determines pixel values withina unit region of an inputted image, the unit region being composed of aplurality of pixels to which a screen threshold matrix is applied, thescreen processor carrying out a screen processing by applying one of aplurality of screen threshold matrices to the plurality of pixels in theunit region based on each determined pixel value, the plurality ofscreen threshold matrices being different in applied positions of thethreshold values from each other.
 17. The image processing apparatus ofclaim 16, wherein the screen processor compares each determined pixelvalue with each threshold value of the screen threshold matrix, andselects one threshold matrix from among the plurality of screenthreshold matrices based on a comparison result to apply the selectedscreen threshold matrix to the screen processing.
 18. The imageprocessing apparatus of claim 17, wherein the screen processor selectsone of the screen threshold matrices in which each threshold value isapplied in an ascending order of the threshold values to the pixels in adescending order of the determined pixel values, based on the comparisonresult.
 19. The image processing apparatus of claim 17, wherein thescreen processor selects one of the screen threshold matrices in which athreshold value smaller than a maximum determined pixel value is appliedto a pixel having the maximum determined pixel value, based on thecomparison result.
 20. The image processing apparatus of claim 16,further comprising: an image determination unit which determines animage attribute of each pixel of the inputted image, wherein the screenprocessor carries out the screen processing by applying one screenthreshold matrix among the plurality of screen threshold matricesdepending on the determined image attribute.